1. Field of the Invention
This invention relates to a radiation image read-out apparatus in a radiation image recording and reproducing system using a stimulable phosphor sheet.
2. Description of the Prior Art
A radiation image recording and reproducing system using a stimulable phosphor sheet is disclosed, for example, in U.S. Pat. Nos. 4,258,264 and 4,315,318, and Japanese Unexamined Patent Publication No. 56(1981)-11395. The system was also announced at meetings of the radiation society, in "Nikkan Kogyo Shinbun" (Daily Industrial Newspaper), June 23, 1981 edition, page 16, and elsewhere, and attracted attention in various fields.
The aforesaid radiation image recording and reproducing system is capable of providing various radiation images useful for viewing, particularly for diagnostic purposes, which could not be obtained by conventional radiography using a silver halide photographic material. Moreover, the radiation image recording and reproducing system can further provide improved diagnostic effects when combined with an up-to-date medical image recording technique such as computed tomography (CT) or introduction of radioisotope into the human body. Therefore, the system is very effective for viewing purposes, particularly for medical diagnosis.
The aforesaid radiation image recording and reproducing system generally carries out the steps of (i) recording (storing a radiation image in a stimulable phosphor sheet), (ii) read-out (converting the stored image into an image signal, and sometimes storing the read-out image signal on a conventional medium such as magnetic tape), and (iii) reproducing (converting the image signal into a visible image, i.e. displaying the visible image on a display device such as a cathode ray tube (CRT) or permanently recording the visible image on a recording medium such as photographic film). The present invention is concerned with the read-out step in the aforesaid radiation image recording and reproducing system.
FIG. 1 is a schematic view showing an example of a radiation image read-out apparatus employed in the aforesaid radiation image recording and reproducing system.
In the apparatus of FIG. 1, a laser beam 1a of a predetermined intensity is emitted as stimulating rays from a laser beam source 1 to a galvanometer mirror 2. The laser beam 1a is deflected by the galvanometer mirror 2 to form a laser beam 1b impinging upon a stimulable phosphor sheet 3 positioned below the galvanometer mirror 2 so that the stimulable phosphor sheet 3 is scanned by the laser beam 1b in the main scanning direction, i.e. in the width direction of the sheet 3 as indicated by the arrow A. While the laser beam 1b impinges upon the stimulable phosphor sheet 3, the sheet 3 is conveyed in the sub-scanning direction as indicated by the arrow B, for example, by an endless belt device 9. Therefore, scanning in the main scanning direction is repeated at an angle approximately normal to the sub-scanning direction, and the whole surface of the stimulable phosphor sheet 3 is two-dimensionally scanned by the laser beam 1b. As the stimulable phosphor sheet 3 is scanned by the laser beam 1b, the portion of the sheet 3 exposed to the laser beam 1b emits light having an intensity proportional to the stored radiation energy. The light emitted by the stimulable phosphor sheet 3 enters a transparent light guide member 4 from its light input face 4a positioned close to the sheet 3 in parallel to the main scanning line. A light guiding mirror 10 having a plane parallel to the main scanning direction is positioned to stand face to face with the light input face 4a. The light guiding mirror 10 reflects the light, which is emitted by the stimulable phosphor sheet 3 towards the light guiding mirror 10, to the light input face 4a, thereby improving the light guiding efficiency of the light guide member 4.
The light guide member 4 has a flat-shaped front end portion 4b positioned close to the stimulable phosphor sheet 3 and is shaped gradually into a cylindrical shape towards the rear end side to form an approximately cylindrical rear end portion 4c which is closely contacted with a photomultiplier 5. The light emitted by the stimulable phosphor sheet 3 upon stimulation thereof and entering the light guide member 4 from its light input face 4a is guided inside of the light guide member 4 up to the rear end portion 4c, and received by the photomultiplier 5. A filter (not shown) transmitting the light emitted by the stimulable phosphor sheet 3 and absorbing the stimulating rays is positioned between the rear end portion 4c of the light guide member 4 and the photomultiplier 5, thereby intercepting the stimulating rays entering the light guide member 4 by being reflected by the stimulable phosphor sheet 3 after impinging thereupon. Therefore, only the light emitted by the stimulable phosphor sheet 3 is guided to the photomultiplier 5. Thus the light emitted by the stimulable phosphor sheet 3 in proportion to the radiation energy stored therein is detected and converted into an electric image signal by the photomultiplier 5. The electric image signal thus obtained is sent to an image processing circuit 6 and processed therein. The electric image signal thus processed is then reproduced into a visible image and displayed, for example, on a CRT 7, or is stored in a magnetic tape 8.
However, in the radiation image read-out apparatus arranged as described above, a problem arises when the stimulable phosphor sheet 3 is scanned by the laser beam (stimulating rays) 1b. Namely, the laser beam 1b impinging upon the stimulable phosphor sheet 3 is reflected by the sheet 3 to the light input face 4a and the light guiding mirror 10, and is further reflected by the light input face 4a and the light guiding mirror 10 to portions of the sheet 3 that have not yet been scanned, thereby stimulating the non-scanned portion and causing it to emit light. When light is emitted by the non-scanned portion of the stimulable phosphor sheet 3 outside of the scanned portion thereof, the light is guided by the light guide member 4 together with the light emitted by the scanned portion of the sheet 3, and is processed as if it were image information from the scanned portion. Therefore, the reproduced image thus obtained becomes incorrect.
FIG. 2 is an enlarged perspective view showing the section near the point 3a of FIG. 1. The aforesaid problem will further be described with reference to FIG. 2 by using as an example the laser beam 1c at a given instant in the scanning by the laser beam 1b in FIG. 1 as an example. The point 3a on the stimulable phosphor sheet 3 exposed to the laser beam 1c emits light in proportion to the radiation energy stored therein upon stimulation by the laser beam 1c. At the same time, the laser beam 1c impinging upon the point 3a is partially reflected and scattered by the point 3a. A part of the laser beam reflected and scattered is further reflected by the light input face 4a of the light guide member 4 and the reflection plane of the light guiding mirror 10 as indicated, by way of example, by arrows 11a, 11b, and 11c, and impinges upon non-scanned portions of the stimulable phosphor sheet 3 outside of the point 3a, thereby stimulating the non-scanned portions and causing them to emit light.